Anti-somatostatin antisera, but neither a somatostatin agonist (octreotide) nor antagonist (CYCAM), attenuates hyperalgesia in the rat

Comparison of Antinociceptive Effect of Octreotide With Morphine in a Rat Model of Acute Inflammatory Pain

Background:

Opioids such as morphine are used for treating moderate to severe pain. However, they also produce adverse effects such as nausea, constipation, addiction, and respiratory depression. Thus, other suitable analgesics need to be identified. Somatostatin is an inhibitory neuropeptide that modulates the transmission of pain. However, the half-life of somatostatin is short. In the present study, the antinociceptive effect of octreotide (a stable long-acting analog of somatostatin) was evaluated in rats with acute inflammatory pain.

Methods:

Sprague Dawley rats (n = 42) were divided into control (n = 6) and carrageenan injected groups (n = 36). The carrageena group was divided into three equal subgroups and treated with saline, morphine (10 mg/kg), and octreotide (3 µg). Rats belonging to each subgroup (n = 12) were again randomly divided into two equal sets. They were subjected to (a) behavioral evaluation of pain (allodynia) and estimation of paw edema, followed by immunohistochemical analysis of the expression of somatostatin type 2 receptor (sst2r) in the spinal cord and (b) estimation of open-field activity. Allodynia and paw edema were measured by von Frey filaments and plethysmometer, respectively, at 3 and 4 h after carrageenan injection. Expression of sst2r was examined after 24 hours, whereas open-field activity was evaluated after 3 hours.

Results:

In comparison to the saline-treated group, allodynia was partially attenuated by octreotide, though this was almost completely reversed by morphine. Paw edema was unaffected by octreotide, though it was marginally increased by morphine. This was not related to increased activity of rats, following relief from pain. Immunohistochemistry revealed a significant increase in the expression of sst2r in saline-treated rats, but a decrease in other groups.

Conclusion:

Octreotide has an antinociceptive effect, which was less than morphine. Increased edema following morphine could result from venodilation. Variations in the sst2r expression suggest its involvement in pain modulation at the spinal level. This information may have clinical relevance.

Effects of somatostatin on the responses of rostrally projecting spinal dorsal horn neurons to noxious stimuli in cats

Somatostatin (SOM) is a widely distributed peptide in the central nervous system and exerts a variety of hormonal and neural actions. Although SOM is assumed to play an important role in spinal nociceptive processing, its exact function remains unclear. In fact, earlier pharmacological studies have provided results that support either a facilitatory or inhibitory role for SOM in nociception. In the current study, the effects of SOM were investigated using anesthetized cats. Specifically, the responses of rostrally projecting spinal dorsal horn neurons (RPSDH neurons) to different kinds of noxious stimuli (i.e., heat, mechanical and cold stimuli) and to the Adelta-and C-fiber activation of the sciatic nerve were studied. Iontophoretically applied SOM suppressed the responses of RPSDH neurons to noxious heat and mechanical stimuli as well as to C-fiber activation. Conversely, it enhanced these responses to noxious cold stimulus and Adelta-fiber activation. In addition, SOM suppressed glutamate-evoked activities of RPSDH neurons. The effects of SOM were blocked by the SOM receptor antagonist cyclo-SOM. These findings suggest that SOM has a dual effect on the activities of RPSDH neurons; that is, facilitation and inhibition, depending on the modality of pain signaled through them and its action site.

Established and potential therapeutic applications of octreotide in palliative care

Octreotide acetate was developed as a pharmacologically stable, long-acting analogue of the hormone somatostatin. Mimicking the actions of somatostatin, octreotide has been used for its antisecretory effects. Randomized control trials have established the efficacy of octreotide for malignant bowel obstruction and for chemotherapy-induced diarrhea. Octreotide has proven to be an effective agent for symptoms of carcinoid syndrome. Newer uses include for bone marrow transplantation, infectious diarrheal syndromes, and management of hepatic metastases. More evidence is needed for the establishment of its efficacy for hypercalcemia, pain, pleural effusions, diarrhea after celiac plexus block, and malignant ascites.

Substance P (NK1) and somatostatin (sst2A) receptor immunoreactivity in NTS-projecting rat dorsal horn neurones activated by nociceptive afferent input

Spinal neurones that receive inputs from primary afferent fibres and have axons projecting supraspinally to the medulla oblongata may represent a pathway through which nociceptive and non-nociceptive peripheral stimuli are able to modulate cardiorespiratory reflexes. Expression of the neurokinin-1 (NK1) receptor is believed to be an indicator of lamina I cells that receive nociceptive inputs from substance P releasing afferents, and similarly, sst2A receptor expression may be a marker for neurones receiving somatostatinergic inputs. In this study, immunoreactivity for these two receptors was investigated in rat spinal neurones retrogradely labelled by injections of cholera toxin B or Fluorogold into the nucleus of the solitary tract (NTS). In addition, nociceptive activation of these labelled cells was studied by immunodetection of Fos protein in response to cutaneous and visceral noxious chemical stimuli. NK1 and sst2A receptors in lamina I were localised to mainly separate populations of retrogradely labelled cells with fusiform, flattened and pyramidal morphologies. Examples of projection neurones expressing both receptors were, however observed. With visceral stimulation, many retrogradely labelled cells expressing c-fos were immunoreactive for the NK1 receptor, and a smaller population was sst2A positive. In contrast, with cutaneous stimulation, only NK1 positive retrogradely labelled cells showed c-fos expression. These data provide evidence that lamina I neurones receiving noxious cutaneous and visceral stimuli via NK1 receptor activation project to NTS and so may be involved in coordinating nociceptive and cardiorespiratory responses. Moreover, a subpopulation of projection neurones that respond to visceral stimuli may receive somatostatinergic inputs of peripheral, local or supraspinal origins.

Octreotide, a somatostatin analogue, attenuates movement evoked discharges of fine afferent units from inflamed knee joints of rats

This electrophysiological study examined whether octreotide, a stable analogue of somatostatin (SOM), reduces the mechanosensitivity of fine primary afferents from inflamed knee joints of rats similarly to SOM itself (Pain 73 (1997) 377). Close intra-arterial application of 200 microl of octreotide (10(-6)-10(-3) M) dose-dependently diminished the responses to passive non-noxious and noxious rotations of the joint in most of the units tested. The inhibitory effects of octreotide required a higher concentration (10(-3) M) compared to SOM to successfully decrease the number of recorded spikes. Application of cyclo-somatostatin, a SOM antagonist, before the octreotide injection prevented the reduction of the movement evoked discharges. These data indicate that octreotide is able to successfully decrease the responses of mechanosensitive fine afferent units innervating the inflamed knee joint of the rat and may, therefore, be useful in the treatment of articular pain of peripheral origin.

Somatostatin directly inhibits substantia gelatinosa neurons in adult rat spinal dorsal horn in vitro

Effects of somatostatin (SST) on the synaptic transmission to substantia gelatinosa (SG) neurons of adult spinal cord slices were investigated using intracellular recording and blind whole-cell patch-clamp technique. Bath application of SST (1 microM) induced the membrane hyperpolarization that was accompanied by a decrease in input resistance and had the reversal potential of -92 +/- 3 mV (n=5) in the intracellular recording experiment. In patch-clamp experiment, SST (1 microM) induced an outward current with amplitude of 14 +/- 2 pA (n=60) at the holding potential of -60 mV, and was not affected by TTX (n=3). The effect was dose-dependent with EC50 value of 0.82 microM (Hill coefficient: 0.89). The outward current was suppressed when the patch-pipette solution containing potassium channel blockers, Cs+ and tetraethylammonium (TEA), and was inhibited by Ba2+ (200 microM) to 15 +/- 6% of the control (n=3). In addition, the SST current reversed its polarity at potential close to the equilibrium potential of K+ channel calculated by the Nernst equation. No significant changes were found in amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs) and dorsal root evoked EPSC (eEPSC) by SST. Also, SST did not affect both of the miniature inhibitory postsynaptic currents (mIPSCs) and evoked inhibitory postsynaptic currents (eIPSCs), mediated by either the GABA or glycine receptor. We conclude that SST activates the K+ channel resulting in postsynaptic hyperpolarization in adult rat SG neurons without affecting presynaptic component of the transmission, which are considered to account, at least a part, for the analgesic effects of SST reported previously.

Modulation of inflammatory paw oedema by cysteamine in the rat

Cysteamine, a potent somatostatin depletor, was used in the present study to investigate the role of endogenous somatostatin in acute peripheral inflammation. The acute inflammation was induced by intraplantar injection of carrageenan (1%), histamine (5 micromol), or formalin (2.5%) in the rat hind paw. The induced inflammation and the formation of oedema were determined by measurement of the paw thickness. Given subcutaneously (s.c.) 1 h before carrageenan, cysteamine caused significant, dose-dependent and long-lasting inhibition of rat paw oedema induced by carrageenan. At doses of 12.5, 25, 50 or 100 mg kg (-1), cysteamine significantly inhibited the carrageenan-induced paw oedema at 4 h by 52.3, 40, 40.7 or 26.3%. Cysteamine given at 300 mg kg (-1), a dose well known to deplete tissue somatostatin, reduced oedema by only 16.2% vs control values. Significant inhibition of the carrageenan-induced rat paw oedema was still evident 24 h post-injection at cysteamine doses of 12.5, 25, 50 or 100 mg kg (-1). Given s.c. at 300 mg kg (-1), 4 h prior to carrageenan, cysteamine decreased rat paw oedema at 4 h by 14.9%. Cysteamine (300 mg kg (-1)), 4 h beforehand, had little modulatory effect on the oedema induced by formalin (2.5%) but reduced that caused by intraplantar histamine (5 micromol). The anti-oedematogenic effect of indomethacin, but not that of the selective COX-2 inhibitor celecoxib, was less marked in rats pre-treated with cysteamine at 300 mg kg (-1). Cysteamine (0.3 microg- 0.3 mg paw (-1)) co-administered with carrageenan was devoid of anti-inflammatory effect and even promoted inflammation at low concentrations. Cysteamine given locally alone induced slight paw oedema. These data indicate that systemic cysteamine possesses potent and long-lasting anti-inflammatory effects and modulates the anti-inflammatory effect of cyclooxygenase inhibitors in a model of peripheral inflammation in the rat. The effect of cysteamine is likely to be mediated via central action.

The epidural and intrathecal administration of somatotrophin-release inhibiting factor: native and synthetic analogues

It is well-known that morphine is the king of analgesics. It is widely used, and administered in various ways for the control of acute and chronic pain states. There are, however, certain types of pain and certain clinical conditions in which morphine cannot be used due to the risk of possible complications. These are usually pain states associated with intracranial hypertension, the presence of serious respiratory problems, the onset of major opioid tolerance, persistent vomiting, and so on. The search for "alternative analgesics" has been in progress for a decade, alternatives that could be used alone or in combination for spinal administration in the treatment of complex chronic pain states and with a low incidence of secondary effects. Today, research is carefully assessing the clinical effectiveness and the side effects of a series of drugs for spinal administration, that is, epidural or intrathecal, such as the new narcotics, alpha-2 agonists, central muscle relaxants, calcitonin, and local anesthetics. In this alternative analgesic category we have to mention the somatotrophin-release inhibiting factor (SRIF), which is an ubiquitous native hormone with widespread, predominantly inhibitory actions, and octreotide, its synthetic analogue. In this article we review the literature on the natural drug and its synthetic analogue, paying particular attention to the problems connected with intraspinal administration and analgesic properties.

Somatostatin inhibits the release of noradrenaline induced by electrical stimulation of the rat mesenteric artery

Somatostatin, a peptide with antisecretory and antiproliferative effects, coexists with noradrenaline in sympathetic neurons. Octreotide, a stable somatostatin analogue, prevents hypertension and cardiovascular structural changes induced by prolonged infusion of DPSPX (1,3-dipropyl-8-sulfophenylxanthine, a non-selective adenosine receptor antagonist) in rats. In the present work we investigated the effect of somatostatin and its analogue octreotide on the release of [(3)H]noradrenaline from sympathetic nerves in the rat mesenteric artery. Rat mesenteric arteries were incubated for 60 min with [(3)H]noradrenaline (0.2 microm), mounted in perifusion chambers, washed out for 90 min and electrically stimulated (2 Hz, 5 min, 50 mA). Radioactivity was measured in the tissue and in the perifusion fluid before, during and after stimulation. Both somatostatin and octreotide inhibited tritium release evoked by electrical stimulation of in vitro preparations of rat mesenteric arteries preloaded with [(3)H]noradrenaline. The maximal effects produced by octreotide and somatostatin were a 56 and 70% inhibition of noradrenaline release, respectively. For somatostatin an EC(50)=0. 18 n m (0.01 n m-2.2 n m;n =16) was calculated. When used alone, the somatostatin receptor antagonist, cyclo(7-aminoheptanoyl-Phe- d -Trp-Lys-Thr[BZL]) (CYCAM; 1 microm), had no effect on noradrenaline release induced by electrical stimulation. However, it was able to significantly antagonize the inhibitory effects of octreotide and somatostatin. These results are compatible with a negative modulatory role of somatostatin on sympathetic neurotransmission.

Spinal modulation of the induction of central sensitization

Peripheral tissue injury results in a change in the excitability of spinal dorsal horn neurons, central sensitization, and the behavioral correlate, hyperalgesia. It is proposed here that a dynamic balance exists between excitatory and inhibitory synaptic input to the spinal dorsal horn that functions to prevent central sensitization following brief, mild, noxious stimulation. Following more severe stimulation and injury, there is a loss of these inhibitory mechanisms that allow central sensitization to proceed. Single-unit recordings were made from L4-L5 deep dorsal horn neurons (wide dynamic range and nociceptive specific) from barbiturate-anesthetized rats that were non-inflamed or had a carrageenan-inflamed hindpaw. Baseline test responses to mechanical stimuli were obtained and normalized to 100%. An electrical conditioning stimulus (1 Hz, 20 s, C-fiber strength) was applied to the tibial nerve or the neuronal receptive field. Five seconds later the test stimulus was repeated and the magnitude of response compared to baseline. During the conditioning stimulus, 46% of the neurons from non-inflamed and inflamed rats showed wind-up although the magnitude of wind-up was significantly greater for inflamed rats. The remaining neurons showed no change (36-46%) or wind-down (8-18%). Five seconds following the end of the conditioning stimulus 67% of the neurons from non-inflamed rats had attenuated responses to mechanical stimuli (36% of baseline). The remaining neurons were either unaffected (30%) or facilitated (3%). Following inflammation significantly fewer neurons (28%) had attenuated responses and the magnitude of attenuation was significantly less than in non-inflamed rats (54% of baseline). The responses of the remaining neurons were unaffected (54%) or facilitated (18%). During subsequent test stimuli, the responses of 30% of the neurons from non-inflamed rats were facilitated to 140% of baseline. The responses of 46% of neurons from inflamed rats were facilitated to 160% of baseline. In these neurons there was significantly less initial attenuation following inflammation compared to non-inflamed rats. The response of the neuron during the electrical conditioning had no effect upon the response following conditioning. The conditioning stimulus given transcutaneously within the receptive field produced qualitatively similar results to tibial nerve stimulation. In non-inflamed rats, when the conditioning/test-stimulus interval was increased from 5 s to 10-30 s, the responses of 20% of the neurons were attenuated (compared to 67%) and the mean magnitude of attenuation was 52% of baseline (compared to 36% of baseline). However, the responses of only 33% of the neurons were ultimately facilitated (compared to 30%). The present study documents a short period following a low-frequency C-fiber input in which the response to natural stimuli is suppressed. It is suggested that this attenuation, whether or not expressed, prevents a significant portion of deep dorsal horn neurons from becoming sensitized to C-fiber input. This functions to prevent central sensitization when the noxious stimulus does not produce inflammation and it is not beneficial to the animal to become hyperalgesic (i.e., to alter its behavior in order to protect an injured limb and reduce painful sensations). Following injury-producing tissue damage and inflammation the mechanisms that produce the attenuation are reduced, with a concomitant increase in excitation to electrical and natural stimuli, suggesting that the attenuation is inhibitory modulation of nociceptive input and injury results in a disinhibition producing an increase in excitability and central sensitization.